Subsea pipes or cables are often laid from pipe or cable laying vessels, where the pipes or cables are stored wound around a large reel before they are laid. FIG. 1 shows an example of a prior art arrangement, where a reel 100 of rigid pipe 102 is mounted on a reel carrier 104. In an alternative prior art arrangement, the reel 100 may be a reel of cable. The pipe 102 is unwound from the reel 100 and passed through a straightener 106 and a tensioner 108 before it moves down a stinger 110 and is laid on the sea bed. The pipe 102 being laid, being a rigid pipe stored on a reel, needs to be straightened and undergo plastic deformation, by means of the straightener 106. Thus, as the pipe 102 is unwound from the reel 100 the straightener 106 bends about a horizontal axis the pipe 102 in order to straighten the pipe 102 from being curved from being wound up around the reel 100. In order that the pipe 102 is not bent about its vertical axis past its limit of elastic deformability, the fleet angle between the pipe 102 and the straightener 106 must be kept below a certain angle. FIG. 2 shows a plan view of the pipe laying arrangement of FIG. 1, where the fleet angle is the angle θ created between the pipe and the straightener due to the offset between the part of the reel 100 from which the pipe 102 is being unwound and the position of the straightener 106. As can be seen, the fleet angle will be greatest when the pipe 102 is being unwound from the edges of the reel 100. When the pipe 102 is directly in line with the straightener 106 the fleet angle is 0 degrees. The fleet angle is always to be considered as a positive angle, as a measure of the deviation away from the 0 degrees when the pipe 102 is directly in line with the straightener 106, regardless of which side of the straightener 106 the pipe 102 has been deviated.
FIG. 2 shows a simplified version of how the pipe 102 travels from the reel 100 to the straightener 106. In most cases, the pipe 102 will actually be controlled so as to form an S-curve in the horizontal plane, with the curvature being kept within the limits of the elastic bending range. This adds further complexity to the pipe laying arrangement. Despite the fact that the pipe 102 may not form a straight line between where it leaves the reel 100 and where it passes through the straightener 106, the fleet angle can still be used as a measure of the offset between where the pipe 102 leaves the reel 100 and meets the straightener 106.
A further disadvantage of having a large fleet angle is that when a pipe or cable is being laid in an arrangement as described above, the large fleet angle may lead to the pipe or cable wearing or becoming damaged where it meets the straightener, leading to failure or reduced working life of the pipe or cable.
In order to keep the fleet angle below a certain maximum angle the reel 100 must be sited a relatively large distance from the straightener 106. In order to reduce this distance slightly, the straightener may be in a “floating” type arrangement, where it is possible for it to move from side to side as necessary and thereby help manage the fleet angle. However, the distance between the straightener 106 and the reel 100 is still large and the straightener arrangement may be complicated.
Also, the straightener must feed the pipe 102 to the tensioner 108 approximately straight on, with no significant fleet angle. Therefore, if the straightener 106 is located closer to the reel 100 due to being in a “floating” arrangement as described, the tensioner 108 must be located further away from the straightener 106 to reduce the change in angle of the pipe it receives due to the movement of the straightener.
The present invention seeks to at least partially mitigate or overcome these disadvantages.